Wednesday, May 4, 2011

Mojave Rattlesnake

The Mojave Rattlesnake (Crotalus scutulatus) is widely distributed in western North America, occurring from the Mojave Desert through the Sonoran and Chihuahuan Deserts to central Mexico. It can be distinguished from other rattlesnake species by features of color pattern and scalation, and especially by the presence of enlarged plates (scutes) on the head between the eyes. There are two distinct venom types known to occur in this species (Wilkinson et al. 1991). Snakes with type A venom have Mojave toxin, a powerful neurotoxin. Mojave RattleSnakes with type B venom lack the Mojave toxin, but have a hemorrhagic toxin instead. Additionally type A + B snakes occur that contain both the Mojave and hemorrhagic toxin.

There are no morphological differences between type A and type B snakes, and the two types apparently belong to the same gene pool. Type B snakes have a restricted geographic distribution; they are found only in central Arizona. Type A snakes are found throughout the remainder of the United States range. Type A+B snakes occur at the contact zone between type A and type B snakes in Arizona. Little is known about the species in Mexico, although Glenn et al. Report three specimens of type A snakes from that country.

Mojave Rattlesnake

The known diet is typical of most-medium sized rattlesnakes: mammals are the primary prey items, with other vertebrates occasionally taken. The most thorough study of mammalian prey in Mojave rattlesnakes is by Reynolds and Scott. These authors examined the stomach contents of mostly road-killed snakes of this and several other species on a transect between Ojinaga and Aldama in Chihuahua, Mexico. They compared the abundance of different rodent species along this transect with the frequency of prey species in snake stomachs. They demonstrated preferences by snakes for particular mammalian prey species.

Their results showed that the most abundant rodent along the transect was Dipodomys merriami; this kangaroo rat was also the most prevalent food item in Crotalus scutulatus. After that, however, these snakes showed preference for prey items that were not necessarily abundant. These included the rodent species Perognathus flavus, Spermophilis spilosoma, and Dipodomys spectabilis; these were over-represented in the snake stomachs compared with their abundance along the transect. In an arena study, C. scutulatus rejected prey items that were too large, too small, or those that were potentially dangerous.

Mojave Rattlesnake

Some Mojave Rattlesnake ecologist have proposed the natural selection influences snake venom composition with respect to the kinds of prey species utilized (e.g. Daltry et al. 1996; da Silva and Aird 2001). This hypothesis predicts that C. scutulatus of different venom types would be feeding on different prey species. Since Type A snakes have a neurotoxic venom that is 30 to 40 times more toxic than B (Glenn et al. 1983), dietary differences between the types could well be profound.

This preliminary study will investigate the diet in C. scutulatus in different parts of its geographic range, including the parts that have different venom types. We are looking for notable dietary differences between the snakes that occur where the type B and type A+B forms are expected, and those snakes from populations that are exclusively type A. If differences are found, additional research in the context of a number of variables will be necessary.

Specimens from the herpetology collections from the University of Texas at El Paso (UTEP) will be utilized. The stomachs of appropriate snakes (e.g. road-kills) will be removed. Stomach contents (if any) will be identified to species level. Live snakes will be examined, by making them regurgitate any prey items they may have swallowed prior to capture. If no prey items are present, then a fecal sample will be taken from the snake, and any undigested contents, such as hair or scales will be removed for identification. Hair samples will from otherwise unidentifiable mammals will be collected and compared with hair samples of known species identity from the mammalogy collections at UTEP. Other vertebrate remains will be identified as necessary. Results will be analyzed with respect to the geographic origin of the specimens and the distribution of venom types.

Mojave Rattlesnake

Diets of most snakes vary with different factors, such as size, age, geographic locality, sex, season, prey availability, etc. The diets of many snakes are thought to have played a significant role in the evolution and distribution of these animals. Dietary information has been used to study the ecology and food habits of different snakes and to see if there is any correlation with different venom types.

Crotalus scutulatus prefer barren, arid grassland-like areas over much of their geographic range they are sympatric with the larger Western Diamondback Rattlesnake, Crotalus atrox . There have been some assumptions made that the two species may be competing for food resources where they occur in the same habitats, but Reynolds and Scott concluded that broad niche overlap between these two might be a regular phenomenon. No study has been made of food competition between C. scutulatus and the similar-sized Prairie Rattlesnake, C. viridis in the Trans-Pecos Texas region where many of our own specimens have originated.

C. scutulatus is mainly a mammal feeder. Klauber recorded mammal remains in the intestinal tracts of twenty-one C. scutulatus and lizard remains in two. He noted that a Mojave rattlesnake ate a leaf-nosed snake while being carried in a sack by a collector. Klauber also found centipede remains, and what appeared to be insects. Immature cottontail rabbits (Sylvilagus) and jackrabbits (Lepus) were found in the stomachs of C. scutulatus by Reynolds and Scott. Our current results have twelve snakes from a sample of forty-five C. scutulatus (from different counties) containing lizard remains. The sample of forty-seven C. scutulatus examined by Reynolds and Scott contained no lizard remains. In contrast to Reynolds and Scott, we have thus far not encountered evidence of lagomorphs in our sample.

The very preliminary and small amount of data collected for this study so far are not adequate for the analysis of intraspecific dietary differences in Crotalus scutulatus. Nevertheless, they suggest that type A Crotalus scutulatus snakes may have a broader diet (a greater proportion of the diet includes lizards over mammals) than type B snakes (mammals predominate). Neurotoxic venoms in snakes are often associated with reptilian prey. Possession of Mojave toxin in populations of Crotalus scutulatus may facilitate the inclusion of a greater proportion of reptilian prey over those that lack the toxin and concentrate their diet on mammals. Additional sampling will be undertaken to test this hypothesis.
Additionally, the hair samples removed from the snake stomachs, as well as the nearly entire rodent remains, will eventually be identified to species level as this study continues. These data will be used to investigate the possibility of fine-grained prey differences among the mammalian prey between type A and type B Crotalus scutulatus.


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